This invention relates in general to antennas, and more particularly, to log periodic antennas.
MIL-STD-464A, entitled “Electromagnetic Environmental Effects Requirements for Systems,” 19 Dec. 2002, and ADS-37A-PRF, entitled “Electromagnetic Environmental Effects (E3) Performance and Verification Requirements,” 28 May 1996, set out stringent electric field requirements for the electromagnetic vulnerability testing of Army helicopters. The antenna generating the test field must generate a test field at levels between 200-264V/m in the 44-150 MHz frequency range, while remaining safe under the helicopter with the rotors turning. Below that frequency range, a whip antenna is used and above that range, standard horn antennas are used. To obtain the required fields, the typical setup includes high power RF amplifiers (10 kW) with heavy duty coaxial cables leading to a log periodic antenna. Typical engineering design for an antenna to efficiently and effectively operate in this frequency range would require a 6 feet long log periodic antenna with the longest elements being about 12 feet in length. When this antenna is turned vertically to create a vertically polarized field, and given a few inches of ground separation, the antenna height would approach 12+ feet. However, standard operating procedures mandate that anything under the rotating helicopter blades must be no more than 6 feet tall. Taking an antenna that is 12 feet tall and reducing the height in half, while still getting the antenna to resonate in the desired test frequency range presents a significant engineering challenge.